1. Field of the Invention
This invention relates to a fuel supply amount control system for internal combustion engines, and more particularly to a fuel supply amount control system which is provided with an evaporative emission control system for controlling the internal pressure of the fuel tank to negative pressure.
2. Description of Related Art
To prevent evaporative fuel generated in the fuel tank of an internal combustion engine installed in a vehicle from being emitted into the atmosphere, there has already been proposed an evaporative emission control system for internal combustion engines, for example, by U.S. patent application Ser. No. 09/021,004, assigned to the assignee of the present application. The proposed system negatively pressurizes the interior of the fuel tank during operation of the engine so as to hold the interior of the fuel tank under negative pressure not only during operation of the engine but also during stoppage of the same, to thereby prevent evaporative fuel within the fuel tank from being emitted into the atmosphere, even if a filler cap of the fuel tank is removed for refueling.
The proposed system includes an evaporative fuel passage extending between the fuel tank and the intake pipe of the engine, a control valve arranged across the evaporative fuel passage, for opening and closing the same, a temperature sensor which detects the temperature of fuel within the fuel tank, and a tank internal pressure sensor which detects the pressure within the fuel tank (hereinafter referred to as "the tank internal pressure"), to set a desired pressure value within the fuel tank to an excessively negative value, i.e. a value lower than the actually required value according to the temperature of fuel within the fuel tank, in view of an expected increase in the tank internal pressure. Further, the opening of the control valve is feedback-controlled in response to an output from the tank internal pressure sensor to control the tank internal pressure by using negative pressure in the intake pipe prevailing during operation of the engine, such that the tank internal pressure becomes equal to the desired pressure value. Thus, the tank internal pressure is normally controlled to and held at the desired pressure value.
In the negatively pressurized fuel tank, immediately after termination of negative pressurization of the fuel tank, volatile ingredients of fuel, which evaporate at temperatures lower than the detected fuel temperature, evaporate due to heat held by the fuel at the fuel temperature, and accordingly the tank internal pressure increases in proportion to the amount of evaporation of the volatile ingredients. The manner of increase in the tank internal pressure is shown in FIG. 1.
As is apparent from FIG. 1, when fuel contains a large amount of low boiling point ingredients, the tank internal pressure increases at a large rate after completion of negative pressurization of the fuel tank, as indicated by a curve A, while when fuel contains a small amount of low boiling point ingredients, the tank internal pressure increases at a small rate, as indicated by a curve B in FIG. 1.
Therefore, by detecting the amount of increase in the tank internal pressure after completion of negative pressurization of the fuel tank, it can be estimated to what degree the volatile ingredients of fuel within the fuel tank have evaporated, i.e. the deterioration degree of the fuel. The deterioration degree means a degree of difficulty of maintaining properties as fuel, which is determined by the degree of evaporation of the volatile ingredients.
In the proposed evaporative emission control system, however, the fuel tank is normally or always held under negative pressure. Therefore, fuel within the fuel tank can become deteriorated with the lapse of time. Supply of the deteriorated fuel, which is poor in volatility, to the engine can cause degraded drivability of the engine especially during a time period after cranking but before completion of warming-up of the engine. Besides, the same inconvenience also occurs where fuel for use in summer containing a small amount of low boiling point ingredients is used in winter.